1. Field of the Invention
The present invention relates to a Global Positioning System GPS and a Global Positioning method for precisely determining a location by receiving GPS signals from satellites.
2. Description of Related Art
Many satellites orbit the earth, and continuously transmit radio waves at a carrier frequency of 1575.42 GHz. The radio waves are phase modulated by pseudo-random sequences, and a unique pattern is assigned to each satellite so that the different radio waves can be easily identified. As a typical pseudo-random sequence, is known a regularly modulated code pattern called a C/A code (clear and acquisition code) available to the public. Furthermore, the radio waves carry navigation data necessary for users to perform positioning, such as satellite orbit information, satellite correction data, correction coefficients of the ionosphere, etc. The navigation data are transmitted by means of polarity inversions in the C/A code sequence.
FIG. 13 is a diagram showing the C/A code sequence. As shown in FIG. 13, the C/A code sequence is a regularly arranged code sequence with its data consisting of 20 PN frames, each which consists of 1023 bits of one millisecond long. Thus, the navigation data is a 50 bit per second signal consisting of 1000 PN frames per second. The polarity of the C/A code sequence is reversed in accordance with the polarity of the bits of the navigation data.
FIG. 14 is a block diagram showing a configuration of a conventional Global Positioning System disclosed in U.S. Pat. No. 5,663,734. In this figure, the reference numeral 101 designates a base station having a GPS receiving antenna 102 and a transmitting and receiving antenna 103. The reference numeral 104 designates a remote unit.
The remote unit 104 comprises an RF (radio frequency) to IF (intermediate frequency) converter 106 with a GPS receiving antenna 105; an A/D converter 107 for converting the analog signal from the converter 106 to a digital signal; a memory (digital snapshot memory) 108 for recording the output of the A/D converter 107; and a general purpose programmable digital signal processor 109 (called DSP from now on) for processing the signal fed from the memory 108.
The remote unit 104 further comprises a program EPROM 110 connected to the DSP 109, a frequency synthesizer 111, a power regulator 112, a write address circuit 113, a microprocessor 114, a RAM (memory) 115, an EEPROM 116, and a modem 118 which has a transmitting and receiving antenna 117, and is connected to the microprocessor 114.
Next, the operation of the conventional GPS will be described.
The base station 101 commands the remote unit 104 to perform a measurement via a message transmitted over a data communication link 119. The base station 101 also sends within this message Doppler information for the satellites in view, which is a form of satellite data information. This Doppler information typically is in the format of frequency information, and the message will specify an identification of the particular satellites in view. This message is received by the modem 118 in the remote unit 104, and is stored in the memory 108 connected to the microprocessor 114.
The microprocessor 114 handles data information transfer between the modem 118 and the DSP 109 and write address circuit 113, and controls the power management functions in the remote unit 104.
Once the remote unit 104 receives a command (e.g., from the base station 101) for GPS processing together with the Doppler information, the microprocessor 114 activates the RF to IF converter 106, A/D converter 107 and memory 108 via the power regulator 112 and controlled power lines 120a-120d, thereby providing full power to these components. This causes the signal from the GPS satellite which is received by the antenna 105 to be down-converted to an IF frequency, followed by conversion to digital data.
A contiguous set of such data, typically corresponding to a duration of 100 milliseconds to one second (or even longer), is stored in the memory 108.
Pseudo range calculation is executed by the DSP 109 that uses a fast Fourier transform (FFT) algorithm, which permits very rapid computation of the pseudo ranges by performing quickly a large number of correlation operations between a locally generated reference and the received signals. The fast Fourier transform algorithm permits a simultaneous and parallel search of all positions, thus speeding up the required computation process.
Once the DSP 109 completes its computation of the pseudo ranges for each of the in view satellites, it transmits this information to the microprocessor 114 through an interconnect bus 122.
Then, the microprocessor 114 utilizes the modem 118 to transmit the pseudo range data over the data link 119 to the base station 101 for final position computation.
In addition to the pseudo data, a time lag may simultaneously be transmitted to the base station 101 that indicates the elapsed time from the initial data collection in the memory 108 to the time of transmission over the data link 119. This time lag improves the capability of the base station 101 to perform position calculation, since it allows the computation of the GPS satellite positions at the time of data collection.
The modem 118 utilizes a separate transmitting and receiving antenna 117 to transmit and receive messages over the data link 119. The modem 118 includes a communication receiver and a communication transmitter, which are alternately connected to the transmitting and receiving antenna 117. Similarly, the base station 101 may use a separate antenna 103 to transmit and receive data link messages, thus allowing continuous reception of GPS signals via the GPS receiving antenna 102 at the base station 101.
It is expected that the position calculations in the DSP 109 will require less than a few seconds of time, depending upon the amount of the data stored the memory 108 and the speed of the DSP 109 or several DSPs.
As described above, the memory 108 captures a record corresponding to a relatively long period of time. The efficient processing of this large block of data using fast convolution methods contributes to the ability to process signals at low received levels such as when reception is poor due to partial blockage from buildings, trees etc.
All pseudo ranges for visible GPS satellites are computed using the same buffered data. This provides improved performance relative to continuous tracking GPS receivers in situations such as urban blockage conditions in which the signal amplitude is rapidly changing.
The signal processing carried out by the DSP 109 will now be described with reference to FIG. 13. The objective of the processing is to determine the timing of the received waveform with respect to a locally generated waveform. Furthermore, in order to achieve high sensitivity, a very long portion of such a waveform, typically 100 milliseconds to one second, is processed.
The received GPS signal (C/A mode) is constructed from a high rate (1 MHz) repetitive pseudo random (PN) pattern (PN frame) of 1023 symbols, and successive PN frames are added to one another. For example, there are 1000 PN frames over a period of one second. The first such frame is coherently added to the next frame, the result added to the third frame, followed by the additions as shown in FIGS. 15(A)-15(E). The result is a signal having a duration of one PN frame (=1023 chips). The phase of this sequence is compared to a local reference sequence to determine the relative timing between the two, thus establishing the pseudo range.
With the foregoing configuration, the conventional Global Positioning System carries out preprocessing operation which precedes the correlation calculations, and which is called "preliminary integration of the received GPS signal" to implement high sensitivity. In this process, the preliminary integration is carried out for 5-10 PN frames to avoid reduction in the integrals due to polarity inversions in the navigation data.
The C/A code sequence in the GPS received signal can change its phases, that is, have polarity inversions at the transitions of the bits of the navigation. Therefore, the signal components (chips) may cancel out each other in the integration (cumulative summing) process because of the polarity inversions at the bits of the navigation data in the C/A code sequence, hindering sufficient improvement in the sensitivity (S/N ratio).
In other words, the conventional system does not detect the polarity inversions in the navigation data.
This limits the theoretical number of data to be integrated, and hence presents a problem of providing only insufficient improvement in the sensitivity (S/N ratio).
In addition, every time it determines its position (called "positioning" from now on), the remote unit functioning as a terminal collects Doppler information from the base station, calculates pseudo ranges to the visible satellites, and determines its position by transmitting the distance information to the server. Thus, the positioning always requires communication with the server, offering a problem of entailing communication cost.